Hitherto, in a seat belt-wearing condition in which a seat belt is worn by a vehicle occupant seated in a vehicle seat, various technologies for blocking occurrence of a phenomenon, i.e., a so-called “submarine phenomenon”, in which the vehicle occupant has a behavior to slip through a downside of a seat belt along a seat surface, along with a frontward movement of a waist portion of the vehicle occupant when an accident occurs, is proposed. For example, in Japanese Unexamined Patent Application Publication No. 5-229378, a vehicle seat having a construction in which an expanded airbag compresses a seat cushion from below so as to block the movement of the vehicle occupant toward a front side of the motor vehicle at the time of occurrence of the accident is disclosed. However, in designing such a kind of vehicle seat, there is a demand for a technology to construct a rational vehicle seat to be aimed at saving weight, and capable of securing strength corresponding to a load caused at a time that the airbag is activated.
Accordingly, it is a problem in the present invention to provide a technology effective for securing strength while saving weight of a vehicle seat on which an airbag module having a construction in which an airbag is developed and expanded from a downside to an upside of a seat cushion against movement of a vehicle occupant in a seating condition in a vehicle seat toward a front side of the motor vehicle when an accident occurs, is mounted.